Many patients with lung or breathing disorders require oxygen enrichment of the air they breathe. This supplemental oxygen is delivered at a constant flow rate to the patient through nasal tubes called cannulae or through face masks. Cannulae are two short pieces of plastic tubing attached to one end of a plastic hose. The two short pieces of plastic tubing fit into the nasal passages. The other end of the plastic hose is attached to the oxygen source. A face mask is a mask which fits over the mouth and nose and is attached to one end of a plastic hose. The other end of the plastic hose is attached to the oxygen source. The most common practice for enriching the concentration of oxygen in the gas inhaled by a patient is to connect the oxygen delivery tubing to a flow meter which is, in turn, connected to the oxygen source. The flow meter allows the flow rate of oxygen, in liters per minute, to be read from a scale on the flow meter and provides a means for selecting and varying the flow rate of oxygen which is delivered to the patient. This most commonly used method of administering supplemental oxygen to a patient provides a constant flow of oxygen and does not provide for monitoring of the patient's breathing cycle, of the presence or absence of oxygen flow, or for automatic control of the flow of oxygen to the patient.
One type of system to supply oxygen to a patient is a demand oxygen system. Cost containment and reduction of the cost of medical care continues to be a major concern both to the federal and state agencies that pay much of the cost of medical treatment and to the public at large who eventually pay all the costs of treatment through taxes, insurance premiums, and direct billings. Although oxygen is only a small part of total medical costs, it is a large and economically significant aspect of in-hospital and at-home medical treatment costs. A demand oxygen system is an oxygen control apparatus which provides oxygen upon demand and not continuously. By providing oxygen on demand, a savings is realized by the hospital and/or patient using the oxygen.
Patient respiration occurs in two phases: inhalation and exhalation. Patients receiving oxygen therapy utilize the oxygen delivered only during the inhalation phase of respiration. No oxygen flow is required by the patients during exhalation. The current practice, however, is to allow the oxygen to flow continuously to the patient. The cost efficiency of providing oxygen only during inhalation is known to those skilled in the art. Methods have been devised to detect respiration and differentiate between inhalation and exhalation. U.S. Pat. No. 4,567,888, issued to Robert et al., on Feb. 4, 1986 describes one such method using a thermistor-triggered electrical valve control to control the oxygen supply during oxygen therapy. The temperature sensing thermistor described in Robert et al. is placed in close contact with the air inhaled and exhaled from the patient. The higher temperature of the air exhaled indicates when exhalation is taking place. The temperature drop sensed when inhalation takes place cues the controller to provide oxygen during the inhalation part of the breathing cycle. In this way, the Robert et al. apparatus is a "demand oxygen system" because it is the function of inhalation which demands the oxygen to be provided to the patient.
Demand oxygen systems are most commonly used in hospitals for patients who require supplemental oxygen to breathe. There are also portable demand oxygen systems for persons with chronic lung ailments. Numerous systems have been devised for monitoring and controlling the flow of oxygen in demand oxygen systems. The quality of the demand oxygen system is directly proportional to the monitoring capabilities of the system. If the person attending a patient is unable to determine the operational status of the demand oxygen system, the demand oxygen system is not doing its job. One drawback of prior art demand oxygen systems is the size required for the systems. In a hospital setting, a large demand oxygen system presents minimal difficulties. In a portable unit, though, it is necessary for the oxygen flow controller to be small.
A patient using a demand oxygen system is able to breathe on his own. In fact, it is the patient's breathing which triggers the operation of the demand oxygen system. When a patient has trouble breathing, a good demand oxygen system should be able to detect such trouble. It is old in the art to detect an irregularity in a patient's breathing. U.S. Pat. No. 4,461,293 issued to Chen describes an apnea event detection device which detects when the patient's breathing has stopped and reacts to dislodge an obstruction from the breathing canal. Most of the prior art devices use a clock and an operator set time after inhalation to look for the next inhalation. If inhalation does not occur within the time permitted, an apnea event is detected. This method of detecting trouble with a patient's breathing has drawbacks. The operator sets the time which determines apnea event detection. This time could vary from patient to patient and also vary depending upon the patient's state: i.e., if the patient is awake or asleep.
It is known in the art to continuously provide oxygen upon the detection of an irregularity in the patient's breathing. When a failure occurs in the control system, though, the prior devices do not provide a continuous flow of oxygen. For example, if the respiration sensor fails, the patient's breathing could not be properly monitored and the demand oxygen system would not properly function. Another hazard in a demand oxygen system is that, since the oxygen flow is controlled by a valve which opens and closes, there is the possibility that the system would malfunction with the valve in the closed position. It is unsatisfactory for the patient to have to advise the operator that he is not receiving any oxygen.